1. Field of the Invention
This invention relates to the transfer of a bovine embryo from a donor cow to a recipient cow. In a preferred embodiment the invention relates to freezing the embryo to preserve it between the time of collection from the donor and implantation in the recipient.
2. Brief Description of the Prior Art
In recent years embryo transfer in cattle has grown rapidly to become a viable component of the animal industry. The primary purpose of embryo transfer is to increase the reproductive capacity of valuable females. With the technique it is possible to utilize genetically superior females, obtain a large number of embryos from them with the techniques of superovulation and artificial insemination, and then disseminate the embryos to other parts of the country and of the world where they can be implanted in local female recipients.
The entire process is optimized when the embryos are rendered in a form suitable for storage. To this end freezing has been utilized. The frozen embryos can be transported over longer distances and into less accessible regions than has been possible with the non-frozen embryos. The transfer of embryos and the use of freezing to render them storable will help to increase animal production. Being able to store the embryos in a state of animation will also greatly facilitate the timing between embryo production and receptivity of the recipient animals.
The practices to date in connection with freezing of embryos have utilized a relatively slow procedure in reducing temperature as a precaution against damaging the cells of the embryo by the freezing process. Typically, temperature reduction has proceeded at the rate of about 1.degree. C. per minute until the embryo reached about -6.degree. C. to -7.degree. C., at which point seeding was executed. Thereafter temperature reduction was limited to a fraction of a degree, such as about 0.3.degree. C. per minute, until the temperature was lowered to about -30.degree. C. Further reduction to about -33.degree. C. was even more slowly executed at a rate of about 0.1.degree. C. per minute, at which time the embryo was plunged into liquid nitrogen. See, for example, "Freezing Mammalian Embryos: A Review of the Techniques", Ralph R. Maurer, Theriogenology, Vol. 9, No. 1, January 1978, pp. 45-68, and in particular, page 58.
It has also been known to use a cryoprotective agent such as dimethyl sulfoxide. Again, in order to avoid adverse affect on the cells of the embryo, particularly osmotic shock which might rupture the cell walls or internal cellular structures, the cryoprotective agent was added through a series of solutions with the embryo being transferred from solution to solution containing increasingly higher concentrations of the cryoprotective agent. Again, the procedure has been time consuming and laborious. Finally, for similar reasons it was thought necessary to remove the cryoprotective agent, such as dimethyl sulfoxide, after thawing the frozen embryo and before implanting it in the recipient animal. The removal was again executed with a series of solutions containing progressively more dilute concentrations of the cryoprotective agent.